Use of Positron Emission Tomography (PET) is growing in the field of medical imaging. In a PET imaging system, a radionuclide is introduced into the object to be imaged. The radionuclide emits positrons. When an emitted positron collides with an electron, an annihilation event occurs, wherein the positron and electron are destroyed. The result of the annihilation event is the production of two gamma rays traveling essentially at 180 degrees from one another. PET imaging systems use detectors positioned across from one another to detect the gamma rays emitting from the object. Typically a ring of detectors is used in order to detect gamma rays coming from each angle.
The collection of a large number of events creates the necessary information for object to be estimated through tomographic reconstruction. Those events, connecting to detector elements, form line-of-response (LOR) that can be histogrammed according to their geometric attributes to define projections, or sinograms to be reconstructed. Events can also be added to the image individually.
The fundamental element of the data collection and image reconstruction is therefore the LOR: a line traversing the system patient aperture. Additional information can obtained on the location of the event. First, it is known that, through sampling and reconstruction, the ability of the system to reconstruct (position) a point is not space invariant across the field of view; better in the center, slowly degrading toward the periphery. The point-spread-function (PSF) is typically used to characterize this behavior. Tools have been developed to incorporate the PSF into reconstruction process. Second, the time-of-flight (TOF), or time differential between the arrival of the gamma ray on each detector involved in the detection of the pair, can be used to limit along the LOR, where the event is more likely to have occurred.
The PSF is three-dimensional in nature and typically varying from an isotropic 4 to 6 mm in the center of the scanner, to highly skewed ellipsoid ranging from 6 to 8 mm at the periphery. TOF information can be converted into a spatial knowledge (through the conversion of the speed of travel of the two gamma). Typically, the TOF information would result in a localization of the event in the order of 10-12 cm down to few centimeters with very fast scanner.
As such, it is desirable to provide a method and means for using all the available information on the LOR during the reconstruction.